Inertial reference systems are commonly employed in aircraft and other vehicular guidance systems. Such systems are packaged within a standard-size box, which is then mounted within the vehicle to be monitored. Sensors within the system detect both attitude and velocity changes with respect to three mutually perpendicular axes. These positional signals are then processed to produce output signals corresponding to positional changes of the vehicle with respect to a fixed, reference position.
In the past, angle rate sensing has been provided by gimballed, spinning wheel gyroscopes. The cost of such gyroscopes has been high, due to the careful assembly required of the precision machine parts employed. In addition, mechanical gyroscopes suffer many limitations, including mass unbalance, structure instabilities, a requirement for extreme cleanliness, and the need for a substantial "spin-up" time prior to use.
Many modern inertial reference systems now utilize ring laser gyroscopes as attitude sensors. The ring laser gyro employs an enclosed light path that is oriented in the sensitive plane of the gyroscope. Two laser beams simultaneously traverse the light path, one in a relative clockwise direction, and the other in a relative counterclockwise direction. When the enclosed path is rotated in inertial space, the clockwise and counterclockwise paths exhibit different lengths. As such, there is a frequency shift between the two traversing light signals, which frequency shift is representative of the rate of rotation of the gyro in its sensitive plane. Thus, by monitoring the difference in frequency of the laser beams traversing the path, an output signal representative of angular rotation rate may be produced. By providing a cluster of ring laser gyros, each with its sensitive plane oriented with respect to three mutually perpendicular axes, attitude changes in any direction may be accurately monitored.
A clearly identified problem with ring laser gyros is a phenomenon known as "lock-in". As the rate of rotation of the gyro is reduced, the two oppositely traversing light rays will frequently lock to a common frequency long before the rotational rate of the gyro has fallen to zero. As a result, resolution of small angular rate deviations is lost.
One way to prevent lock-in is to dither the gyroscope in its sensitive axis. By applying a sufficient dither motion to the gyroscope, the two oppositely traversing light beams constantly experience rate changes sufficient to prevent the two beams from locking to a common frequency.
In inertial reference system applications, the three ring laser gyros forming a cluster are all mounted to a common structure, designed to orient each gyro in one of the three mutually perpendicular axes. If all ring laser gyros are dithered synchronously, this could result in the excitation of resonant frequencies of the gyros and support structure, thereby producing undesired deflections and corresponding error output terms from the gyros. As such, it is imperative that the three ring laser gyros be dithered asynchronously.
The signal produced by a ring laser gyro is digital in nature, having a pulse repetition rate corresponding to the rate of gyro angular displacement. To eliminate the dither signal from the produced gyro signal, the count of the gyro pulse output is accumulated over one full dither cycle, and measured at dither zero crossings. Inasmuch as the three gyros are dithered asynchronously, the corresponding gyro dither zero crossings occur asynchronously. The navigational algorithms used to process the data and produce positional output information assume, however, that the rate data for each of the three mutually perpendicular inertial reference system axes is taken at the same point in time. Thus, to avoid errors in the produced positional data for the inertial reference system, a means must be provided to resynchronize each of the three gyro signals to a common interval.
Heretofore, inertial reference systems employing ring laser gyros have been relatively large in size, and quite expensive in cost. As such, the systems lend themselves to application only in larger aircraft. It is desirable to reduce the size and cost of ring laser gyro-based inertial reference systems such that they are suitable for a broad application for all sizes of aircraft or other vehicles. As the ring laser gyros and accelerometers are packaged in smaller enclosures, special attention must be paid to the temperature compensation of the sensors, both at an individual sensor level, as well as a system level. Also, bias and offset compensation both at a sensor and a system level should be provided, along with sculling correction for the accelerometers and coming correction for the gyroscopes. Further, the correction factors used for temperature, bias, and other error source correction must be capable of inexpensive and high-speed recalibration, thereby minimizing the downtime of such devices, while maximizing accuracy. Further, the functions of the inertial reference system should be modularized in such a way that a single basic inertial reference system configuration may be quickly and inexpensively modified for use in any of a variety of applications.